Light intensifier circuit for flash photography



Jan. 14, 1969 B. J. SULLIVAN 3,422,308

LIGHT INTENSIFIER CIRCUIT FOR FLASH PHOTOGRAPHY Filed July 14. 1966 28 I34 ZZKA- l 2| IOOKFD BERNARD J. SULLIVAN INVENTOR.

ATTORNEY United States Patent 2 Claims ABSTRACT OF THE DISCLOSURE Alight intensifier circuit includes a first circuit for continuouslyenergizing a lamp from an alternating current power source for lowintensity output and a second circuit for momentarily energizing thelamp for high intensity output with a rectified alternating signal for aduration of integral half cycles of the alternating current power.

The invention relates to electrical circuits in general and moreparticularly to an electrical circuit for controlling the intensity of alight source for producing at least two levels of intensity.

When photographs are made through the use of optical instruments such asmicroscopes, endoscopes, and retinal cameras, etc., it is desirable toprovide a continuous source of illumination for viewing, focusing, etc.,and also intense illumination for a short period of time duringphotographic exposure. It is difficult, if not impractical, to employtwo separate light sources due to the necessity of strict opticalalignment required in these instruments. Furthermore, it is impracticalto substitute one type illuminant for another, particularly if the subject is continually moving and it is desirable to observe the subjectwhile, or shortly before, photographing.

'In the light intensifing apparatus of the prior art, a single lamp orlight bulb was generally energized at a low voltage level while acapacitor was charged to a substantially higher voltage. At a precisetime, the capacitor is discharged through the lamp to provide an intensesource during photographic exposure. In such cases, the intensity of theflash is a function of the amount of energy stored in the capacitorandthe impedance match between the capacitor acting as a source and thelamp as a load. The duration of the flash is determined by theelectrical time constant of the capacitor-filament combination and thethermal time constant of the lamp filament. The size of the capacitoremployed generally changes with the characteristic of the particularlamp connected in the circuit. Accordingly, various lamps can not bereadily interchanged, or several connected in parallel withoutcompensating therefore by changes in the size of the dischargecapacitor. In addition, the capacitor employed in such light intensifiercircuits is in the order of hundreds or thousands of microfarads. Thistype of capacitor is large and expensive, particularly if the capacitoris to be charged to a relatively high voltage. Furthermore, thecapacitor is generally rapidly or impulsively discharged, subjecting thecapacitor to severe operating conditions that may cause an overloadcondition and damage after a short period of service.

It is therefore an object of this inventor to provide a new and improvedlight intensifier circuit.

It is also an object of this inventor to provide a new and improvedlight intensifier circuit for momentarily increasing the light intensityof a lamp without the use of a large storage capacitor as a source ofenergy for intensifying the output lamp.

-It is also an object of this invention to provide a ice new andimproved light intensifier circuit adapted to provide a means forcontinuously energizing a lamp at a low level and momentarily energizesthe lamp at a high level for a predetermined time duration.

-It is still a further object of this invention to provide a new andimproved light intensifier circuit for momentarily increasing theintensity of a lamp for a time duration determined by the period of ahalf cycle of alternating signal applied thereto.

It is also an object of this invention to provide a new and improvedlight intensifier circuit that readily allows for the interchange oflamps or connection of lamp in parallel.

The intensifier circuit of the invention includes a pair of inputterminals adapted to be connected to an alternating current power sourceand a pair of output terminals adapted to be connected to a lamp to beenergized. Circuit means are coupled between the input terminals and theoutput terminals for applying a potential to the output terminals havinga value within the rating for the continuous energization of the lamp.Switching circuit means are also provided coupled between the inputterminals and the output terminals for applying a substantially higherpotential to the output terminal for a time duration for at least onehalf of a cycle of the alternating current power source so that the lampprovides an intense source of radiation at a desired time.

The novel features which are considered to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation as well as additional objects and advantages thereof, willbest be understood from the following description when read inconnection with the accompanying drawings in which the figureillustrates an embodiment of an intensifier circuit including theinvention.

Light intensifier circuit of the figure includes a filament type lamp 10that may be suitably mounted in an optical apparatus for providing afirst level of illumination for the viewing, focusing, etc., and also asecond higher level of intensity for a short duration for photography.The intensifier circuit includes a pair of terminals 14 adapted to beconnected to an alternating current power source (not shown such as the60 cycle line mains. A primary winding 16 of a transformer 18 isconnected to the input terminals 14. The transformer secondary windingincludes a first winding portion 20 (between the terminals 21 and 27)and a second winding portion 22 (between the terminals 21 and 39). Theturns ratio in the winding portions 20 and 22 is such that asubstantially greater voltage is developed across the winding portion 22than across the winding portion 20.

The filament of the lamp 10 is connected across a pair of outputterminals 11 and 12. A low level energizing potential of a magnitudewithin the voltage rating for the continuously energizing lamp 10 isdeveloped by the series circuit including the winding portion 20' andthe diode 24 connected across the terminals 11 and 12. The lamp 10 iscontinuously energized at this low level to provide the low intensitylight source required for viewing, focusing etc.

A switching circuit is inclined in the intensifier circuit to provide ahigh intensity energizing potential across the terminals 11 and 12 of acontrolled time duration energizing the lamp 10* for producing a highintensity light flash. The switching circuit includes a controlledrectifier 26 having its anode and cathode electrodes connected in aseries circuit between the terminals 27 of the first winding portions 22and the output terminal 12. The controlled" rectifier is adapted to berendered conductive when its anode is positive with respect to thecathode and is extinguished when the polarity of the voltage reverses.By controlling the duration of the firing pulses applied to thecontrolled rectifier gate electrode the number of half cycles of pulsesof conduction is controlled.

A firing circuit for the controlled rectifier 26 includes series.circuit including a diode 28, a resistor 30, and a capacitor 32connected across the first winding portion 22. The diode 28 functions tocharge the capacitor 32 to a direct current potential. The junction ofthe resistor 30 and the capacitor 32 is connected through a switch 34and diode 35 to the controlled rectifier gate electrode and also to adischarge resistor 36 connected in a series with the diode 38 to aterminal 39 of the second Winding portion 20. A suitable filtercapacitor 37 is connected between the secondary terminal 21 and thejunction of the diode 38 and the resistor 36 to provide a substantiallyconstant voltage for maintaining the diode 35 nonconductive (keeping thecontrolled rectifier cut off) until the switch 34 is closed. The switch34, may for example, be mechanically coupled to a shutter of camera,remotely located, or be a semi-conductor circuit for periodically firingthe controlled rectifier 26 at a desired pre determined rate.

When the switch 34 is closed, the capacitor 32 discharges through theresistor 36, the diode 38 and the capacitor 37 to generate a voltagepulse at the gate electrode of the controlled rectifier 26- ofsufiicient amplitude to render the controlled rectifier conductive. Ifit is desired to limit the conduction of the controlled rectifier 26 toone half cycle of the applied alternating current, the resistor 36 andthe capacitor 32 R-C time constant is set to be in the order of theperiod of one cycle of the applied alternating current or less. If it isdesired to fire the controlled rectifier 26 for several consecutive halfcycles, the R-C time constant is increased accordingly, The valuesillustrated in the figure allow a single half cycle of current flowthrough lamp when the switch 34 is momentarily closed. The ratio of theresistor 36 over the sum of the resistors 30 and 36 is selected so thatthe capacitor 32 charges at a rate so that the switch 34 may becontinuously depressed and the lamp 10 is not energized for a greaternumber of consecutive half cycles of current pulses than desired. Forexample, with the values of the components in the figure, the lamp 10 isenergized for only one half cycle when the switch 34 is continuouslyclosed.

As previously mentioned, the controlled rectifier 26 is renderedconductive :for at least a half cycle or a plurality of half cycles (asdetermined by the R-C time constant of the capacitor 32 and the resistor36) to apply a high potential of a controlled duration across the lamp10, causing the lamp to produce a light of high intensity (flash). Itshould be noted that the duration of the high potential applied to thelamp 10 is not determined by a time constant including the filament ofthe lamp (such as the capacitor and filament combination of the lamp asin the prior art) but rather on the frequency of the applied energizingalternating current potential and a number of integral half cycles ofconduction. Furthermore, any number of lamps can .be connected inparallel with the lamp 10 (within the capacity of a transformer 18 andthe controlling rectifier 26) Without substantially changing the timeduration of the flash or the amplitude of the high potential applied tothe lamps.

In addition to the foregoing, the value of the capacitor 32, asindicated, is 100 microfarads which is small compared to that of thestorage capacitor of the prior art generally requiring substantiallygreater values.

I claim:

1. The electrical circuit comprising:

a pair of output terminals adapted to 'be connected to a lamp forenergization thereof;

a pair of input terminals adapted to be connected to an alternatingcurrent source of power;

first circuit means coupling said input terminals to said outputterminals to provide an energizing potential for continuously energizingsaid lamp,

control means including first and second terminals exhibiting acontrollable unidirectional current path therebetween and a controlterminal for rendering said paths conductive;

second circuit means connecting said first and second terminals of saidcontrol means between said input terminals and said output terminals toprovide a substantially higher potential to said output terminals in theform of a rectified alternating current signal when said path isrendered conductive, and

third circuit means, including a switch coupled to said control terminalfor rendering said path conductive for at least a portion of one halfcycle of said alternating current power when said switch is activated.

2. The electrical circuit as defined in claim 1 wherein:

said first means includes a transformer including a first windingcoupled to provide an energizing potential that is less than thepotential of said alternating current source.

3. The electrical circuit as defined in claim 2 wherein:

said second circuit means includes a second winding on said transformerproviding a substantially higher alternating current potential than saidfirst winding.

4. The electrical circuit as defined in claim 1 wherein:

said control means comprises a cont-rolled rectifier;

said second circuit means connects said controlled rectifier betweensaid input terminals and said output terminals, and said third circuitmeans includes a firing circuit for said controlled rectifier circuit,and means for coupling said firing circuit to said controlled rectifierto render said controlled rectifier conductive for at least one half ofa cycle of said alternating current source.

5. The electrical circuit as defined in claim 4 wherein:

said means for coupling said controlled rectifier to said inputterminals includes a transformer, and

said controlled rectifier being connected in a series circuit betweensaid transformer and said output terminals.

6. The electrical circuit as defined in claim 4 wherein:

said firing circuit comprises a capacitor and a rectifying circuitcoupling said capacitor to said input terminals for developing acharging potential for said capacitor, and

said means for coupling said firing circuit to said controlled rectifiercomprises a switch coupled to said controlled rectifier for applyingsaid charging potential thereto for rendering said controlled rectifierconductive.

7. A light intensifier circuit comprising:

a lamp;

a transformer adapted to be connected to a sixty cycle power source;circuit means coupling said transformer to said lamp for applying apotential thereto having a value within a rating for the continuousenergization of said lamp;

a switching circuit coupled between said transformer and said lampadapted to be actuated for a period of integral half cycles of saidsixty cycle power source for applying a substantially higher potentialto said lamp in the form of a rectified alternating current signal.

8. A light intensifier circuit as defined in claim 7 wherein saidswitching circuit includes a controlled rectifier connected in a seriescircuit between said transformer and said lamp.

9. A light intensifier circiut as defined in claim 8 wherein saidswitching circuit includes a charging circuit and means for connectingsaid charging circuit to said control rectifier for rendering saidcontrolled rectifier conductive for a duration of at least one halfcycle of said sixty cycle source.

10. A light intensifier circuit as defined in claim 9 wherein saidcharging circuit includes a capacitor and rectifying circuit connectedin series with said transformer for charging said capacitor to apredetermined voltage,

and means for connecting said charging circuit to saidcontrolledrectifier includes a switch for applying said voltage between the gateand cathode electrodes for said controlled rectifier for rendering saidcontrolled rectifier conductive.

11. A light intensifier circuit as defined in claim wherein saidcharging circuit comprises a series circuit including a diode, acapacitor and a resistor connected across a secondary winding of saidtransformer for developing a charge across said capacitor, and saidmeans for connecting said charging circuit to said controlled rectifiercomprises a switch and a resistor connected in a series circuit betweensaid capacitor and a secondary winding of said transformer so that whensaid switch is closed, said charge is applied to gate electrode of saidcontrolled rectifier to render said controlled rectifier conductive.

12. A light intensifier circuit as defined in claim 11 wherein saidcapacitor is discharged through said series circuit including saidswitch and said resistor sufiiciently within the period of one cycle ofsaid applied alternating current power source whereby said controlledrectifier conducts for a time duration in the order of the period of onehalf cycle of said applied alternating current.

References Cited OTHER REFERENCES A Shutter-Synchronized Circuit ForTransiently Brightening an Incandescent Lamp Applicable To ColorPhotomicrography: by Edwin Gordy and GeorgefS. Siever, Published inPhotographic Science and Engineering, vol. 8, No. 5, September-October1964 (copy available in Scientific Library).

US. Cl. X.R.

JOHN W. HUCKERT, Primary Examiner.

R. F. POLISSACK, Assistant Examiner.

